I did some kickboxing drills in college club. We did slips, but I'm trying to incorporate head rolls from YouTube into my home regimen. To me, it seems that one has to be superhumanly fast to see a strike being initiated and respond appropriately with a head roll. Does one really roll the head in after seeing an incoming strike? Or does one simply do the rolls as "evasive manuvering", hoping to be hard to hit?
The question is whether head rolling in boxing is something done in response to seeing the strike coming or just done preemptively, without knowing there's a strike coming.
The answer is both. It's actually pretty fast and natural to see something coming and turn your head the other way. This is the way people react even without any boxing training whatsoever. It's like flinching.
But usually reaction times are on the order of a half a second to 3/4 of a second for most people when it comes to blocking a strike. Whereas a head roll might take a quarter of a second to a half a second to perform as a reaction. There are many punchers that can reach your head before that time runs out. But, chances are good that if you're the type of person that takes a whole second to react to a punch, you're not going to appear on TV or in any boxing matches. Why? You'll be washed out before you start. You just aren't elite enough.
So when you see boxers on TV, know that these guys are already kind of super human. They have very low reaction times. The sport has selected them. Only the top 0.01% are going to reach that level. (I just made that statistic up, but you get what I mean.)
There's also preemptive head rolling. If you can read your opponent well enough and have studied his patterns, you can guess what he's going to do next, before he does it. That gives you a head start (no pun intended). You can begin doing a head roll and evasion before or just as he starts to strike. It looks like they have almost no reaction time whatsoever. But in truth, they're just good at this.
To improve reaction times, you can pre-load this technique in your mind, so it's ready to use. The more you have to think about what to do, the more time it takes to react. So having it right on the tip of your brain will help your reaction time. Just be careful not to start doing it too often, because your opponent can see that you're doing that and may use that against you.
Hope that helps.